As a vehicle is being driven, various road, weather, and traffic conditions are encountered. Those conditions change as the vehicle is traveling toward its destination and to correct for changes, power which is delivered to the vehicle's drive wheels is varied by the driver's actions and/or automatically by one or more controllers to adjust operation of the prime mover and/or components in the vehicle's drivetrain. For example, particular driving conditions, such as travel up a grade, typically call for the prime mover to deliver increased power to the drive wheels while other conditions, such as travel down a grade, typically call for the prime mover to deliver decreased power.
An internal combustion engine is one example of a prime mover of a vehicle. Two common types of internal combustion engine are a spark-ignited gasoline engine and a compression-ignition diesel engine. Other types of engines use natural gas or propane as fuel. In an electric powered vehicle, the prime mover is an electric motor/generator operating as a motor to propel the vehicle while drawing current from a battery bank.
The performance of an internal combustion engine can be rated in different ways using parameters such as output power, output torque, and engine speed, for example by a torque/speed curve and a power/speed curve which show peak output torque and peak output power, which may not necessarily occur at the same engine speed. Peak torque and peak power are however commonly used to rate performance of an engine.
Torque is commonly measured in pound-feet (lb-ft), and power in horsepower (hp). Speed is typically measured in crankshaft revolutions per minute (rpm). Because vehicles operate over a range of various driving conditions, engine speed, engine torque and engine power will, as mentioned above, vary depending on those conditions. So too will engine fuel efficiency.
An engine may be selected for use in a particular vehicle based on performance which is capable of providing acceptable vehicle operation for a maximum load which is expected to be imposed on the engine. An estimate of expected maximum load is based on various factors such as the size, weight, and load-carrying capacity of the vehicle, and/or the velocity which the vehicle should attain when fully loaded and travelling up a specified grade. Aerodynamic resistance may also be taken into account. A properly sized engine should have a maximum power rating for delivering enough power to handle the expected maximum load.
However because maximum power is typically used only intermittently during vehicle travel along a road such as an interstate highway, the engine will at other times operate at less than maximum rated engine power because load on the engine is less than the expected maximum load which calls for maximum rated engine power.
An internal combustion engine is inherently more fuel-efficient when operating in a zone of an engine torque/speed map which provides greater engine power than when operating in a zone which provides lesser engine power. However, when fuel economy of a vehicle is a factor in the selection of an engine for use in a particular vehicle, and it is decided to use an engine having smaller maximum rated power instead of one having larger maximum rated power, vehicle performance is sacrificed.